This mission was simultaneously a great achievement and a horrible failure. Soyuz 11's prime crew was replaced by the backup crew four days before launch after medical tests showed a member of the prime crew might have tuberculosis. After the backup crew arrived in orbit, they performed the first successful docking to the world's first space station, Salyut 1. The crew became the first and only people to enter, live, and work aboard Salyut 1. After a three-week stay on the station, Soyuz 11 undocked and deorbited to come home after 23 days, a record at the time. As the Soyuz is three separate craft and only the Descent Module safely reenters, it separated via pyrotechnics shortly after the deorbit burn. When this happened on Soyuz 11, a ventilation valve on the top of the Descent Module was shaken open at 168 km instead of inside Earth's atmosphere just before landing. Soyuz 11's oxygen gradually leaked out. As the Soyuz program did not use pressure suits at the time, the crew suffocated to death before reentry. Georgy Dobrovolsky, Vladislav Volkov, and Viktor Patsayev are the only people to date to die in space. Soyuz 11 remains the second and most-recent fatal Russian in-flight space accident.